A digital microscope makes a digital image of a sample. Often this is done by repeatedly scanning up and down and stitching different bands together and/or by overlaying images measured at different wavelengths. For an accurate, artefact-free image it is important that the various image pieces line up accurately. In a line scanning system, where the sample is scanned with a constant velocity in one direction, while a line sensor measures information in the perpendicular direction, two axes can be defined: the scan direction and the lateral direction. Two main sources of errors are firstly variations in the scan velocity and secondly a non-straightness of the travel of the sample. The variations in the scan velocity result in errors in the scan direction. This type of error can be corrected by measuring the position of the stage in the scan direction and triggering the acquisition of the line camera at well-defined and equidistant positions. The non-straightness of the travel of the sample results in errors in the line sensor direction. Depending on the type of stage, the non-straightness is between nanometers and many microns. The degree of non-straightness mainly depends on the bearings used. For most microscopy applications the absolute straightness is less of an issue than the reproducibility. For artefact-free stitching/overlay it is important that the shift between consecutive scans is less than half of a pixel pitch (pixel spacing) in the image. One object of the invention is to provide a device and a method that can be used to compensate errors in the line sensor direction caused by variations in the non-straightness of the travel of the stage. Further, it is an object of the invention to provide a device that has relaxed requirements on the travel accuracy of the stage. In principle, many of these errors could be corrected in post-image processing steps. But, for applications where high data rates are needed and large files are generated post-processing means are very calculation-intensive and time-intensive. Thus, it is preferred to solve these problems directly online. In lithography systems and in optical storage systems similar problems occur. In U.S. RE38,113 E a system is described which interferometrically measures the deviation of a scanning substrate perpendicular to a scan movement. This signal is used to move the sample with an actuator on an axis perpendicular to the direction of the scan movement. Another means of measuring deviation is disclosed in U.S. Pat. No. 7,079,256B2 which describes a system that functions as a non-contact height profiler. Optical storage devices are disclosed in W02005/106857A1 and W02007/054884A2 where marks on an information carrier can be interrogated by the readout device in order to correctly position the sample in two dimensions. In these conventional systems the correcting or positioning is done by moving the stage. Such a conventional scanning microscope has a complex structure, a moderate speed, and low cost-efficiency.
It is an object of the present invention to provide a simpler scanning microscope having higher speed and higher cost-efficiency than the conventional scanning microscope. This object is solved by providing a scanning microscope according to the independent claim.